The present invention relates to drive systems for switched inductive loads, such as motor windings, relay coils, and the like. The invention is particularly useful for driving multi-phase stepping motors, and is therefore described below with respect to that application.
Multi-phase stepping motors are increasingly being used in applications requiring the accurate positioning of a member in response to a train of pulses. Various types of such stepping motors are known. Generally, they include a plurality of phase windings energizable by the train of pulses according to a predetermined switching sequence for rotating the output shaft in discreet angular steps.
One of the drawbacks of such stepping motors, however, is the limitation of speed. This limitation stems from the high inductance of the phase windings preventing the rapid current build-up in the windings as they are sequentially switched-on. That is to say, when a phase winding of the stepping motor is switched on, the current rises according to the exponential L/R, where L is the inductance and R is the resistance of the winding. The speed of operation may be increased by increasing the resistance R which lowers the time constant of the circuit, but this also increases the power requirement. A number of other methods have been employed for improving the speed performance of such drive circuits, but these too have not been found satisfactory.